Diffractive Optical Element Asymmetric Exit Pupil Expansion
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Solution Overview
Problem
Retinal scanning-type image display apparatuses face issues with vignetting and reduced light intensity due to the small diameter of the light beam, leading to a darker image and potential stray light problems when using an exit pupil expander to increase the exit pupil diameter.
Innovation Solution
A scanning-type image display apparatus incorporating a diffractive optical element that increases the divergent angles of the light flux in two orthogonal directions, optimizing the shape and size of the exit pupil to enhance light usage and reduce light loss, while preventing stray light by adjusting the periodic structure of the diffractive optical element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of stationary object
If an exit pupil expander is used to increase the exit pupil diameter, then the exit pupil size is improved, but the image brightness deteriorates due to reduced light amount reaching the observer
Solution Approach 1:
The patent applies asymmetry by using a diffractive optical element with different periodic structures in orthogonal directions (first and second directions). This creates an asymmetric exit pupil shape where the diameter in the first direction is larger than in the second direction, allowing optimized light distribution that maintains brightness while expanding the exit pupil in the critical direction for observer comfort.
Solution Approach 2:
The patent changes the optical parameters by using a diffractive optical element that differently controls light divergence in orthogonal directions. The element increases the divergent angle of light flux in the first direction more than in the second direction, thereby expanding the exit pupil asymmetrically while preserving sufficient light intensity for bright image observation.
2Area of stationary object
If an exit pupil expander is used to increase the exit pupil diameter, then the exit pupil size is improved, but stray light increases making image observation difficult
Solution Approach 1:
The asymmetric periodic structure of the diffractive optical element controls stray light by directing diffracted light primarily in the desired first direction while minimizing diffusion into other directions. This asymmetric light control expands the exit pupil without creating excessive stray light that would degrade image quality.
3Speed
If a small scanning apparatus is used to achieve fast scanning and high resolution, then the scanning speed and resolution are improved, but the light beam diameter becomes very small causing vignetting
Solution Approach 1:
The patent addresses the small light beam diameter issue by expanding the beam in the exit pupil plane through the diffractive optical element. While the scanning apparatus maintains its small size for fast scanning, the diffractive element effectively increases the light beam diameter in the exit pupil area by controlling light divergence, thereby preventing vignetting without compromising scanning speed.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution allows for a larger exit pupil size in the desired direction, improving image observation and reducing light loss, resulting in a brighter image with minimized stray light and optimized light usage.
Implementation Method 1
a diffractive optical element which receives the converged light flux from the second optical system... the diffractive optical element has a function of increasing the divergent angles of an emerging light flux from the diffractive optical element
Data Source
AI summary
A scanning type image display apparatus is disclosed which is capable of forming an exit pupil with a shape and a size which facilitate observation. The apparatus includes a scanning unit, a first optical system introducing a light flux to the scanning unit, a second optical system converging the light flux from the scanning unit, a diffractive optical element receiving the converged light flux, and an ocular optical system. Two directions in diametral directions of the light flux are first and second directions. The optical element increases the divergent angles of an emerging light flux therefrom in the first and second directions as compared with the convergent angles of an incident light flux thereinto in the two directions and increases the divergent angle of the emerging light flux in the first direction as compared with the divergent angle thereof in the second direction.